MIPS: Kconfig: Drop the EXPERIMENTAL tag from MIPS R6

The MIPS R6 ISA support has been part of mainline since v4.0
and it should be in a good shape nowadays so it is not an
experimental feature anymore.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10731/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>